The present invention relates generally to etching methods and more particularly, to a method for etching Gallium Nitride (GaN) materials to produce GaN semiconductor devices.
Gallium Nitride (GaN) is a wide band gap material which possesses desirable properties for wide band gap semiconductor devices including high breakdown voltage and current handling capability. When implemented in a high power amplifier, the high breakdown voltage property of the GaN material translates into a higher power, higher efficiency amplifier when compared to similar frequency, same device periphery Gallium Arsenide (GaAs) amplifiers. The high GaN power density means that the equivalent frequency, same topology, GaN amplifier will occupy a reduced material area, approximately 100 times smaller, than the equivalent frequency GaAs amplifier. Thus, significant performance improvements can be realized from GaN devices. However, to realize the improvement from GaN devices, the device must be finely dimensioned and precisely etched. This has proven to be difficult using standard dry and wet chemical etching techniques.
Dry etching techniques have proven to be inadequate for etching GaN material since these techniques irreparably damage the GaN surface. Although high ion density plasmas have shown reduced damage when compared to lower ion density plasmas, these low damage dry etches are still highly damaged when compared to a wet chemically etched GaN surface.
Wet chemical etching provides better results than dry chemical etching for GaN material but still lacks the control necessary to precisely etch the GaN material. The current wet chemical etch technique consists of immersing the material in an etching solution and illuminating the material with ultraviolet (UV) light. The UV light induces a reaction in the material which causes atoms of the material to change from a solid phase to a liquid phase and dissolve in the etching solution thereby etching the material. This technique does not provide sufficient control to precision etch a GaN material as the primary control element is the intensity of the light and the material quality of the GaN material.
The shortfall in the prior art wet etching technique is extremely evident when it is used to etch a GaN heterojunction bipolar transistor (HBT). In etching an HBT, it is very important to etch the material in a manner which will minimize the roughness of the etched surfaces. The roughness of the etched surface is related to the diffusion of ions from the surface of the material. With the prior art wet etching method, the roughness of the etched surfaces is dependent on the uniformity of the light, the concentration of the etching solution at any point, and the quality of the material to be etched. A non-uniform illumination of the material will result in a rough surface. In addition, poor quality or defected material will result in electron hole pairs recombining at the places of the defects and etching will occur around the defects leaving intact the undesirable defect. This is particularly a problem for GaN materials since typical GaN materials contain a substantial number of impurities and defects.
In addition to providing a smooth surface after etching, to form an HBT having an emitter/base/collector from n-p-n GaN material, it is very important to etch away the entire preselected portion of the emitter material and stop the etching process almost exactly at the surface of the base material so that a contact can be placed directly on the base. If too much of the emitter material remains over the base, the contact will not allow for conduction in the base layer. And, if too much of the surface of the base material is removed, the electrical properties of the HBT will be degraded. The present wet etching technique depends on the intensity of the light to control the etch depth which does not provide sufficient control to stop the etching at an exact point. Therefore, when applied to n-p-n GaN material, the prior art wet etching method typically over etches or under etches the material resulting in a poor performing HBT.
What is needed therefore is a method for precision etching of GaN material which provides for improved surface smoothness and is operable to selectively etch n-p-n GaN material to produce GaN HBT's.